Polar bears live only in the Arctic and depend on sea ice to hunt seals
The United States has listed the polar bear as a threatened species, because its Arctic sea ice habitat is melting due to climate change.
US government scientists predict that two-thirds of the polar bear population of 25,000 could disappear by 2050.
However, the government stressed the listing would not lead to measures to prevent global warming.
Environmentalists have expressed disappointment that more will not be done to protect the bear's habitat.
US Interior Secretary Dirk Kempthorne said the government had made the decision on the advice of scientists, but he suggested the impact of the move would be limited.
"While the legal standards under the Endangered Species Act compel me to list the polar bear as threatened," he said, "I want to make clear that this listing will not stop global climate change or prevent any sea ice from melting."
He said that could only be achieved through action by all of the world's major economies.
Mr Kempthorne also said he was taking measures to make sure the listing was not "abused" to make policies on climate change which would cause "harm to the society and the economy of the United States".
A federal judge had ordered the US government to make a decision on the issue by May 15.
Environmental campaigners described the listing as a limited victory.
"Protecting the polar bear under the Endangered Species Act is a major step forward," said Andrew Wetzler of the Natural Resources Defense Council in a statement.
"But the Bush administration has proposed using loopholes in the law to allow the greatest threat to the polar bear - global warming pollution - to continue unabated," he continued.
Polar bears in their natural habitat
Mr Kempthorne said there would be greater steps to monitor polar bear populations in Alaska, and more cooperation with foreign governments to protect the species.
But environmentalists said this would not be enough.
"By denying a direct link between the sources of global warming pollution and the loss of the polar bears' sea ice habitat, and by denying that the polar bear will be protected from oil and gas development, they're willing to sit by and let the polar bear go extinct," said John Kostyack of the National Wildlife Federation.
In February, the Bush administration sold drilling rights for oil and gas off the Alaskan coast, which includes an area of polar bear habitat.
Canada - home to around 15,000 polar bears - has not listed the animals as threatened.
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